The use of hydroxyaromatic chloroformates as a chainstopping or endcapping agent in polycarbonate synthesis facilitates control of the molecular weight of the polycarbonate to be formed. In general, the greater the quantity of endcapping agent introduced into a polycarbonate synthesis, the lower the molecular weight of the polycarbonate product. Hydroxyaromatic chloroformates are particularly suitable as endcapping agents in interfacial polycarbonate synthesis because they produce a polycarbonate product in a single step phosgenation with a lower level of diarylcarbonate (DAC) than products produced using a hydroxyaromatic endcap, such as p-cumyl phenol. Diarylcarbonates have a low melting point, and a polycarbonate containing diaryl carbonates can lead to undesirable effects, such as "plate out" in which the DAC from previous molding cycles condenses on the mold and leads to blemishes in subsequent molding. The term "DAC" as used herein is understood to include also di(alkylphenyl carbonates) and di(arylphenyl)carbonates.
U.S. Pat. No. 5,399,657 (Van Hout et al) discloses a method of preparing hydroxyaromatic chloroformate compounds in which a solution of phosgene in a solvent is introduced into a reactor, to which phosgene and a phenol compound are then added while maintaining the temperature at the desired level by cooling, and maintaining the pH at desired levels by the addition of an aqueous caustic solution.
U.S. Pat. No. 5,274,164 (Wettling et al) discloses a method of preparing aryl chloroformates by the reaction of phenols with phosgene in the presence of organic phosphorous compounds. The addition of a catalyst, such as the organic phosphorous compounds, necessitates extra process steps to recover the catalyst from the product.
U.S. Pat. No. 4,864,011 (Bussink et al) discloses a method of preparing an aromatic polycarbonate with a phenolic chloroformate chain stopper.
It would be desirable to develop a process in which hydroxyaromatic chloroformates are produced in an interfacial process from one or more monofunctional hydroxyaromatic compounds, a carbonate precursor, an aqueous caustic solution and an organic solvent with a minimal amount of DAC contaminants and unreacted monofunctional hydroxyaromatic compound. Such a process would allow production of a polycarbonate product having superior properties.